Soldering is a well-developed art which is widely used to join various metals to each other. Many solders are available commercially, many of them combinations of low-melting metals such as tin and lead. Metal solder, generally available in wire form, is melted and applied over the two metals to be joined. When the solder cools, it solidifies, encapsulating the joint. In order to provide a good bond, the solder must adhere well to both of the joined surfaces. This requires that the metal surfaces to be joined must be clean and that the solder will wet the surfaces of both metals well. Fluxes are often used during the soldering process to clean the metal surfaces and to improve surface wetting by the molten solder. In fact many commercial solders includs a flux core. Fluxes usually include organic resins, organic acids or salts or inorganic acids and salts. During bonding, the flux first displaces air and moisture on the surface to be bonded, i.e., wets the surface, and then removes any surface tarnish or oxide from the surface of the metal. This can be done by a chemical reaction, such as reducing the surface oxide back to metal, or by dissolving the surface oxide. The solder then displaces the flux on the surface and surrounds the joint.
Solder bonding for metals produced by electrolysis, such as aluminum and magnesium, has several problems. Aluminum forms a comparatively thick, tenacious oxide coating on its surface immediately upon exposure to air or oxygen which is very difficult to remove with conventional fluxes. Thus special fluxes are often used, such as organic fluorides, often combined with heat-stable salts such as cadmium fluoroborate, metal fluorides, inorganic chloride and ammonium compounds. These fluxes are very corrosive, however, and they are temperature-sensitive. Zinc chloride or tin chloride are also used as fluxes for aluminum, but the temperature of use must be carefully controlled.
Another problem with soldering both aluminum and magnesium is their high electromotive potential relative to that of most solders. This difference in potential leads to severe corrosion of the joint when exposed to moisture in the atmosphere and results in failure of the soldered joint.
However, it is highly desirable to use magnesium and aluminum metals and their alloys for applications in space due to their light weight. For example, eliminating one pound in the manufacture of a satellite reduces the power requirements of the spacecraft by a significant amount. Up until now the use of aluminum or magnesium sheet to which a plurality, sometimes hundreds, of soldered wire bonds must be made, has not been feasible because of the poor bond strengths obtained. In general bond strengths of over 10 pounds, preferably over 20 pounds, are required for these applications.